Tractor-scrapers used in earthworking typically operate under three conditions; loading, transporting and unloading, all usually accomplished while the vehicle combination is moving. Suspension requirements vary for these conditions. During the transporting phase, the prime requirement is for cushioning of road shocks imparted to the vehicle, either loaded or unloaded and damping of bounce resulting from such road shocks. During the loading phase it is important to maintain the scraper and scraper bowl, where the load is carried, at a desired level to insure an even cut of earth. Road shock suppression, or cushioning, is relatively unimportant because of the slow speed of the vehicle combination. As earth is received in the scraper bowl, the weight of the load is constantly increasing and consequently, without a lockup means, the scraper bowl and scraper may move to a lower position in relation to the tractor axle, the scraper then taking a progressively deeper bite of earth as the load increases. It has been found that during this loading phase it is also desirable to permit a degree of side-to-side rotation about the longitudinal axis of the vehicle, or roll oscillation, for better cutting action. During unloading the scraper bowl will move upward because of the loss of weight. Although the bowl position is not critical as in the loading phase, if the suspension system were not leveled, the subsequent unloaded condition with the suspension system adjusted for a full load would result in little or no shock cushioning during the return trip for a subsequent load.
There are several types of suspension systems available on the market today for an earthworking tractor-scraper vehicle. The two most generally accepted types of suspension systems for tractor-scrapers are either the suspended cushion hitch or the suspended axle. The suspended cushion hitch depends upon ride cushioning means incorporated into the hitch connecting the tractor and scraper while the axles are rigidly affixed to the tractor frame and the scraper frame. The suspended axle suspension system utilizes a suspension arm pivotably connected at one end to the tractor frame and carrying at the other end of the axle. Suspension cylinder struts are affixed between the axle end of the suspension arm and the frame. Means for cushioning road shock may be included with such suspension cylinders. Either type of suspension system offers characteristics desirable in an earth-working tractor-scraper to improve the rideability of the vehicle. Typical of the suspended axle system are characteristics which do not provide for synchronized leveling of the vehicle while loading and unloading, nor permit the vehicle to roll when the system is locked up for loading purposes.
Typical leveling systems for vehicle suspension are disclosed in U.S. Pat. Nos. 3,784,228 and 3,618,972. Neither of these patents provide for locking-up or isolating the cushioning qualities of the suspension system. U.S. Pat. No. 3,539,197 discloses a trim corrector for insuring that the vehicle frame is maintained in a level status while damping out transient signals received by the trim corrector due to acceleration and deceleration. Although these three patents deal with leveling devices, none of them address the problem of this invention, i.e., locking out the ride cushioning capability while providing limited roll oscillation. U.S. Pat. Nos. 3,118,686 and 3,135,529 address suspended axle wheel suspension systems but do not address a load leveling feature or a roll oscillation feature. U.S. Pat. Nos. 3,311,389 and 3,321,216 (assigned to the assignee of this invention) provide leveling means in suspended cushion hitch tractor-scraper combinations but do not address a roll oscillation feature.